JUBA, Friday

US Secretary of State John Kerry flew in to war-torn South Sudan on Friday in an effort to bring about a ceasefire and halt the country's slide towards genocide and famine.

The unannounced visit is seen as the most determined push yet for a truce in the four-month-old civil war, which has seen the world's youngest nation collapse amid a brutal cycle of war crimes including widespread ethnic massacres, rape and child soldier recruitment.

On arrival in Juba, Kerry headed into a meeting with President Salva Kiir, an AFP correspondent said.

US officials said he would also be holding telephone talks with rebel leader Riek Machar, and brandishing the threat of targeted sanctions against both sides in the conflict.

"Secretary Kerry will reiterate the need for all parties to respect the cessation of hostilities agreement (and) to immediately cease attacks on civilians," State Department spokeswoman Jen Psaki said.

REBEL MASSACRE

He will also urge warring factions to "fully cooperate with the United Nations and humanitarian organisations to protect civilians and to provide life-saving assistance to the people of South Sudan," Psaki added.

Outrage is mounting over the scale of killings in South Sudan, with both government forces loyal to President Kiir and rebels backing ex-vice president Machar implicated in a string of atrocities.

Last month hundreds of people were massacred by rebels in the northern oil-hub of Bentiu -- including in churches, mosques and hospitals -- while a pro-government mob shot dead dozens of unarmed civilians sheltering in a UN base in the town of Bor.

The United States has also been under pressure to intervene, having been a key backer of South Sudan's push for independence from Khartoum and having poured in billions of dollars in aid to the country since it split from Sudan in 2011.

Kerry has signalled that Washington has lost patience, saying on Thursday that he was "frankly disappointed" by the conduct of both Kiir and Machar, whose bitter rivalry has sparked a wave of ethnic killings between their respective Dinka and Nuer tribes.

DANGER OF GENOCIDE

Peace talks taking place in Ethiopia have made no progress, and a ceasefire signed in late January was almost immediately violated by both sides.

Speaking in the Ethiopian capital Addis Ababa on Thursday, Kerry cited "very disturbing leading indicators of the kind of ethnic, tribal, targeted nationalistic killings taking place."

"Were they to continue in the way they've been going (they) could really present a very serious challenge to the international community with respect to the questions of genocide," he added.

President Barack Obama signed a decree last month authorising punitive sanctions, including the seizure of assets and visa bans, against anyone in South Sudan deemed to be threatening peace efforts.

A State Department official said Kerry had also met with African Union officials in Addis Ababa earlier Friday and discussed "the scope of atrocities in South Sudan and the need to hold people accountable."

Thousands of people have already been killed -- and possibly tens of thousands -- with at least 1.2 million people forced to flee their homes. Tens of thousands are living in appalling conditions in overstretched UN bases.

UN APPALLED

Aid agencies are also warning that South Sudan is on the brink of Africa's worst famine since the 1980's, with the United Nations demanding at least a one-month-long truce so that crops can be planted and food stocks boosted.

On Wednesday the UN High Commissioner for Human Rights, Navi Pillay, visited South Sudan and issued a similarly bleak assessment of the situation, saying she was "appalled by the apparent lack of concern about the risk of famine displayed by both leaders."

"The deadly mix of recrimination, hate speech, and revenge killings... seems to be reaching boiling point," Pillay said.

The conflict started on December 15, with Kiir accusing Machar of attempting a coup. Machar then fled to the bush to launch a rebellion, insisting the president had attempted to carry out a bloody purge of his rivals.